Even if you bought your books used to begin with, keep them in as good of a condition as possible (like new if you can). This means no high-lighting, writing in or especially no ripping out pages. The better the condition your books are in the better chance you have to get more money back for them. Treat them with care.
Get estimates. Sometimes the bookstore that wants to buy back your books may actually have a reasonable price. If so, it could save you the work of finding other means of trying to sell your books. Go around to all the bookstores that our offering book buy back (or check on their websites) and compare the prices. If you think the buy back price is too low then move on to step 3.
Make fliers advertising the books that you are trying to sell. This way if there is anyone taking the course that requires your text book they can contact you if they're interested. However, here's the catch: Find out how much textbooks cost for the new semester and make sure that your sell price does not go higher than that price. For example; if a book was selling at a book store for $80, it wouldn't make sense for you to try to sell your book for $90 or so on. Balance the budget that you want to sell between the cost at the book store and the cost that you bought it for. This way you and the person buying your book can have a win-win situation.
If you don't find anyone interested in buying your book in person or around your campus you can finally try eBay. The same principle of balancing the price you sell the book for applies here. Make sure to take a good, clear photo of your book and be honest with the describing the book condition.